Holographic digital data storage system compatible with a CD/DVD player

ABSTRACT

The holographic digital data storage system is compatible with a CD/DVD player. A polarization direction of the beam is altered by a first wave plate and the size of the polarized beam is expanded by a beam expander. The expanded beam is split into a vertical polarized beam and a horizontal polarized beam by a polarization splitter; a polarization direction of the horizontal polarized beam is altered by a second wave plate. An optical path of the vertical polarized beam is adjusted by a first mirror to generate a first path-controlled beam; an optical path of a third directional polarized beam is controlled by a second mirror to generate a second path-controlled beam; the first and the second path-controlled beams are concentrated by using a first and a second lenses, respectively; and the interference of two beams is recorded on a medium.

FIELD OF THE INVENTION

[0001] The present invention relates to a holographic digital data storage system; and, more particularly, to a holographic digital data storage system compatible with a CD/DVD player.

BACKGROUND OF THE INVENTION

[0002] Recently, there are reported increasing levels of active researches on holographic digital data storage systems as semiconductor lasers, charge coupled devices (CCDs), liquid crystal displays (LCDs) and the like are being developed. Since the holographic digital data storage system normally features a large storage capacity and high data transfer rate, it has already been applied to, e.g., fingerprint recognition systems for storing and reproducing fingerprints, and the scope of its applications keeps expanding.

[0003] The holographic digital data storage system allows a signal beam transmitted from an object to interfere with a reference beam, and writes an interference pattern generated from such interference phenomena on a storage medium such as a crystal or a photopolymer which reacts differently depending on an amplitude or phase of the interference pattern. In the holographic digital data storage system, the phase of the signal beam as well as the amplitude thereof may be recorded by changing an incidence angle of the reference beam, so that a three dimensional display of an object can be realized. Further, hundreds to thousands of hologram digital data constituted with binary data on a page-by-page basis can be stored in a single space of the storage medium.

[0004]FIG. 1 depicts an overall block diagram of a holographic digital data storage system, wherein the holographic digital data storage system comprises a light source 20, a beam expander 21, a beam splitter 22, two reflection mirrors 23 and 24, a spatial light modulator (SLM) 25, a medium 26 and a CCD 27.

[0005] The light source 20 generates an optical signal, e.g., a laser beam, whose wavelength falls within a specific wavelength band required for the holographic digital data. The beam expander 21 expands the size of the laser beam.

[0006] The beam splitter 22 separates the expanded laser beam into a reference beam and a signal beam and transfers the reference beam and the signal beam through two different transmission channels, wherein the reference beam and the signal beam correspond to a transmitted beam and a reflected beam, respectively.

[0007] The reference beam is reflected at the reflection mirror 24 so that the reflected reference beam is transferred to the medium 26. The signal beam, on the other hand, is reflected at the reflection mirror 23 so that the reflected signal beam is transferred to the SLM 25. The SLM 25 modulates the reflected signal beam into binary pixel data on a page basis. The modulated signal beam is transferred to the medium 26. In case the reflected signal beam is, for example, image data provided on a frame basis, the reflected signal beam is preferably modulated on a frame basis and the reflection mirror 24 functions to change the reflection angle of the reflected reference beam by a small amount.

[0008] The medium 26 stores the interference pattern acquired from an interference phenomenon between the reflected reference beam and the modulated signal beam, wherein the interference pattern depends on the reflected signal beam, i.e., the data inputted to the SLM 25. In other words, the modulated signal beam irradiated to the medium 26 has been modulated on a page basis and the reflected reference beam has been reflected in an angle corresponding to the modulated signal beam. The modulated signal beam interferes with the reflected reference beam within the medium 26. The amplitude and phase of the interference pattern results in a photo-induction within the medium 26 so that the interference pattern may be written on the medium 26.

[0009] When only the reference beam is irradiated onto the medium 26 in order to reconstruct the data written thereon, the reference beam is diffracted by the interference pattern within the medium 26 so that a check pattern with original brightness on a pixel basis may be restored. When the check pattern is irradiated on the CCD 27 in turn, the original data may be restored. The reference beam used for reproducing the data written on the medium 26 should be irradiated at the same incidence angle as that of the reference beam when recording the data on the medium 26.

[0010]FIG. 2 presents a block diagram of a conventional CD or DVD player, wherein the CD/DVD player comprises a high frequency overlap module 10, two mirrors 11 and 18, a polarizing prism 12, a cylindrical lens 13, an photodiode (PD) 14, a λ/4 plate 15, a disc medium 16, an object lens 17 and a collimating lens 19. A detailed description for the structure and the operational principle of such CD/DVD player will be omitted here since it is well known to a person having ordinary skill in the relevant art.

[0011] As for the conventional CD/DVD player of FIG. 2 and the conventional holographic digital data storage system of FIG. 1, however, there has been found a drawback in that they cannot be compatible with each other since the positions of their detectors, e.g. optical diodes, are different from each other. To be specific, since the CD/DVD player has its detector at a direction of reflection while the holographic digital data storage system has its detector at a transmission direction, a single detector cannot be used for both systems. Further, the size difference of beams used in the two systems is so great that two different optical instruments are required.

SUMMARY OF THE INVENTION

[0012] It is, therefore, a primary object of the present invention to provide a holographic digital data storage system compatible with a CD/DVD player by using a mirror-coated medium for the holographic digital data storage system and changing optical channels and optical sizes by controlling a a spatial light modulator (SLM) in the holographic digital data system such as a micro-mirror array or a transmissive liquid crystal display (transmissive LCD).

[0013] In accordance with a first preferred embodiment of the present invention to achieve the above-stated goal, there is provided a holographic digital data storage system compatible with a CD/DVD player, comprising:

[0014] a light source for generating a beam;

[0015] a first wave plate for altering a polarization direction of the beam to a predetermined direction to generate a polarized beam;

[0016] a beam expander, disposed to confront with the first wave plate, for expanding the size of the polarized beam;

[0017] a polarization splitter for splitting the expanded beam into a vertical polarized beam, vertical to the polarization direction, and a horizontal polarized beam, horizontal to the polarization direction;

[0018] a second wave plate for altering a polarization direction of the horizontal polarized beam to another predetermined direction to generate a third directional polarized beam;

[0019] a first mirror for adjusting an optical path of the vertical polarized beam to generate a first path-controlled beam;

[0020] a second mirror for controlling an optical path of the third directional polarized beam to generate a second path-controlled beam;

[0021] a first and a second lenses for concentrating the first and the second path-controlled beams, respectively, to generate a first and a second concentrated beams; and

[0022] a medium for recording the interference of the first and the second concentrated beams.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given with conjunction with the accompanying drawings, in which:

[0024]FIG. 1 is a block diagram of a conventional holographic digital data storage system;

[0025]FIG. 2 depicts a block diagram of a conventional CD/DVD player;

[0026]FIG. 3 presents a block diagram of a holographic digital data storage system compatible with the CD/DVD player in accordance with a first embodiment of the present invention;

[0027]FIG. 4 describes a principle of compatibility of the holographic digital data storage system shown in FIG. 3;

[0028]FIGS. 5A to 5B demonstrate a writing/reconstruction principle of the holographic digital data storage system shown in FIG. 3;

[0029]FIG. 6A explains an operational principle of a micro-mirror array shown in FIG. 3 and an optical path thereby;

[0030]FIG. 6B shows an operational principle of a micro-mirror array in the case of using the optical expander shown in FIG. 3 and an optical path thereby;

[0031]FIG. 7 sets forth a block diagram of a holographic digital data storage system compatible with a CD/DVD player in accordance with another embodiment of the present invention;

[0032]FIG. 8A describes an optical path through a spatial light modulator (SLM) shown in FIG. 7;

[0033]FIG. 8B describes an optical path at a time when the SLM shown in FIG. 7 is shifted; and

[0034]FIG. 9 provides a block diagram of a holographic digital data storage system compatible with the CD/DVD player in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0035]FIG. 3 is a block diagram of a holographic digital data storage system compatible with a CD/DVD player in accordance with an embodiment of the present invention, wherein the holographic storage system comprises a light source 300, two wave plates 302 and 310, an optical expander 304, a polarization splitter 306, a charge coupled device (CCD) 308, a mirror 312, a micro-mirror array 314, two lenses 316 and 318 and a medium 320.

[0036] The light source 300 is an essential element for the writing and reconstruction process of the holographic digital data storage system. A laser, for example, can be used as the light source. The light source 300 provides an optimum wavelength band for the medium 320 of the holographic digital data storage system. An available wavelength band depends on a photo-sensitizer and an initiator added to the medium 320.

[0037] The wave plates 302 and 310 are installed between the light source 300 and the optical expander 304 and between the polarization splitter 306 and the micro-mirror array 314, respectively, to rotate a polarization direction of an incident laser beam by a predetermined degree.

[0038] The optical expander 304 expands the beam size of the incident laser beam to generate an expanded beam. The polarization splitter 306 splits the expanded beam into a vertical polarized beam, vertical to the polarization direction, and a horizontal polarized beam, horizontal to the polarization direction.

[0039] The mirror 312 reflects the vertical polarized beam to form an optical channel in a predetermined direction. The micro-mirror array 314 including a number of mirror pixels can reflect the incident horizontal polarized beam into a desirable direction selectively by controlling each of the mirror pixels. The micro-mirror array 314 in accordance with the present invention may be used to adjust the numerical aperture in the CD/DVD player and also be used as a spatial light modulator (SLM) for the holographic digital data storage system.

[0040] The two lenses 316 and 318 are used to concentrate the laser beams reflected from the mirror 312 and the micro-mirror array 314, respectively.

[0041] The medium 320 for the holographic digital data storage system in accordance with the present invention is characterized in forming a mirror coating 324 and then coating a holographic digital data storage material 322 on the mirror coating 324. In other words, the holographic digital data storage system compatible with the CD/DVD player can be implemented by forming the mirror coating 324 between a substrate 326 and the holographic digital data storage material 322.

[0042]FIG. 4 describes a principle of compatibility between the CD/DVD player and the holographic digital data storage system. It is assumed that the beam is irradiated through the lens with a focal length F at a predetermined angle a. A beam factor B_(F) of the CD/DVD player should be constant so that the holographic digital data storage system and the CD/DVD player are compatible. In general, B_(F) of the CD player is 0.5769 □m⁻¹ and the B_(F) of the DVD player is 0.9230 □m⁻¹ The B_(F) can be calculated as follows: $\begin{matrix} {B_{F} = \frac{N.A.}{\lambda}} & {{Eq}.\quad 1} \end{matrix}$

[0043] wherein λ and N.A. represent a wavelength of the beam and a numerical aperture, respectively. When different wavelength is used, the N.A. is controlled in such a way that the B_(F) remains constant and thus the CD/DVD player can be played. The N.A. is calculated as follows:

N.A.=n·sin α  Eq. 2

[0044] wherein n represents a refractive index of a material after a beam passes through the lens and α represents a concentration angle against an optical axis, i.e., a central axis, of the lens in case an incident beam vertical to the lens is concentrated on a focus. In other words, sin α is a function of the focal length F of the lens and a beam width W of the beam incident into the lens given as follows: $\begin{matrix} {{\sin \quad \alpha} = \frac{W}{2\sqrt{\left( \frac{W}{2} \right)^{2} + F^{2}}}} & {{Eq}.\quad 3} \end{matrix}$

[0045] Accordingly, the beam width W can be derived from an equation as follows: $\begin{matrix} {W = {\frac{2F\quad B_{F}\lambda}{n} \cdot \frac{1}{\sqrt{1 - \left( \frac{B_{F}\lambda}{n} \right)^{2}}}}} & {{Eq}.\quad 4} \end{matrix}$

[0046] Consequently, the B can be sustained at a constant value by controlling the beam width W and thereby adjusting the N.A., so that the CD/DVD player can be played

[0047] When an Nd-YAG laser beam having a wavelength λ of 532 nm is transmitted through the air whose refractive index is 1 and a lens with a focal length E of 1 cm is employed, a beam factor B_(FCD) for the CD player and a beam factor B_(FDVD) for the DVD player are 0.5769 μm⁻¹ and 0.9230 μm⁻¹, respectively. Accordingly, the beam widths W_(CD) and W_(DVD) required in the CD/DVD player are calculated as follows respectively: $\begin{matrix} {W_{CD} = {\frac{2\left( {1{cm}} \right)\left( {0.5769\quad \mu \quad m^{- 1}} \right)\left( {0.532\mu \quad m} \right)}{\sqrt{1 - {\left( {0.5769\mu \quad m^{- 1}} \right)^{2}\left( {0.532\mu \quad m} \right)^{2}}}} = {0.64495{cm}}}} & {{Eq}.\quad 5} \\ {W_{DVD} = {\frac{2\left( {1{cm}} \right)\left( {0.9230\mu \quad m^{- 1}} \right)\left( {0.532\mu \quad m} \right)}{\sqrt{1 -}\left( {0.9230\mu \quad m^{- 1}} \right)^{2}\left( {0.532\mu \quad m} \right)^{2}} = {1.12734{cm}}}} & {{Eq}.\quad 6} \end{matrix}$

[0048] The beam width W_(CD)/W_(DVD) can be adjusted by controlling an on/off operation of each mirror pixel within the micro-mirror array 314. When the CD or DVD player is used, only a predetermined range of beam proceeds to the medium 320 by controlling all the mirror pixels in the micro-mirror array 314.

[0049]FIGS. 5A and 5B illustrate a writing/reconstruction principle of the holographic digital data storage system in accordance with the present invention. In the holographic digital data storage system, the holographic digital data is conventionally recorded in a writing mode by using the interference between the reference beam and the signal beam. Herein, the signal beam should enter the lens through a predetermined portion of the lens, because otherwise the incident beam interferes with the reflected beam reflected from a surface of the mirror coating 324, resulting in a noise.

[0050] A reconstructed beam in a reconstruction mode of the holographic digital data storage system proceeds along the direction of the original signal beam. Accordingly, the reconstructed beam having the same form as the reflected beam of the signal beam is outputted in the reproducing mode.

[0051] The followings are detailed description of the operational principle of the holographic digital data storage system in accordance with the present invention.

[0052] The rotation angle of the wave plate 302 changes the polarization direction of a laser beam outputted from a light source 300 to generate a polarized beam while the laser beam passes through the wave plate 302. The polarized beam above is composed of a vertical polarized beam and a horizontal polarized beam, wherein the intensities of the reference and the signal beam depend on the component ratio of the vertical and the horizontal polarized beam. If the polarization direction of the laser beam passed through a wave plate is λ/8, the intensity of the reference beam equal to that of the signal beam. The beam expander 304 expands the polarized beam and then the polarization splitter 306 divides the expanded polarized laser beam into a vertical polarized component and a horizontal polarized component. In the present invention, it is assumed for illustration that the vertical polarized component is regarded as a vertical polarized beam which is transmitted through the polarization splitter 306 and the horizontal polarized component as a horizontal polarized beam which is reflected at the polarization splitter 306 (a polarization splitter having the reverse constitution may also exist).

[0053] The transmitted horizontal polarized beam passes through the wave plate 310 and becomes a vertical polarized beam having a vertical polarized component as the signal beam. The signal beam subjects to the micro-mirror array 314 such as a TMA (thin-film micro-mirror array). Specifically, by controlling the mirror pixels in a predetermined range for the holographic digital data storage system, the signal beam may be modulated into a modulated signal beam that corresponds to the required signal. The modulated signal beam is irradiated into the medium 320 through the lens 318. FIG. 6A demonstrates an operational principle of the micro-mirror array 314 and an optical channel of the beam around there.

[0054] An optical expander 400 may be added between the micro-mirror array 314 and the lens 318 in order to obtain a higher resolution and to store more data. FIG. 6B describes an operational principle of the micro-mirror array 314 and the optical path thereby in case the optical expander 400 is used. When the optical expander 400 is utilized, however, a distortion by an aberration and the like can be generated as the modulated signal beam becomes more distant from the optical axis of the lens 318.

[0055] The vertical polarized beam, which is a reference beam, is reflected at the mirror 312 and irradiated into the medium 320. In the above step, when a shift multiplexing method, as shown in FIG. 3, is used, the lens may be adjusted so that the vertical polarized beam is focalized beforehand and then illuminated to the medium 320 as the signal beam. In case another multiplexing method is adopted, an adequate module therefor should be added. For example, in case an angular multiplexing method is utilized, a unit such as a Galvano mirror may be preferably added.

[0056] As described above, data can be written on a holographic digital data storage material 322 by using the interference between the reference beam and the signal beam.

[0057] The reconstructed beam, reconstructed by the above-stated process, passes through the lens 318. The region of the lens 318 where the reconstructed beam passes through is a region of the lens 318 which had not been used in the writing process of the signal beam. The reconstructed beam passes through the lens 318 and then proceeds to the micro-mirror array 314. After the direction of the reconstructed beam is changed at the micro-mirror array 314, the reconstructed beam proceeds to the wave plate 310. The wave plate 310 adjusts the polarization direction of the reconstructed beam such that the reconstructed beam should be reflected at the polarization splitter 306. The reflected reconstructed beam is displayed on the CCD 308.

[0058] When the holographic digital data storage system is used with a CD/DVD player, on the other hand, the first wave plate 302 may be preferably adjusted in such a way that only the horizontal polarized beam (the component which transmits the polarization splitter) exists. The beam is expanded by the beam expander 304 and the expanded beam is transferred to the polarization splitter 306. The beam which is transmitted through the polarization splitter 306 is controlled so that the polarized beam rotates by as much as λ/4 by the second plate 310. The beam is modulated by the micro-mirror array 314 so as to have a beam size adequate for the CD or DVD player. Afterwards, the beam passes through the lens 318 and is irradiated into the medium 320 on which records are written for the CD/DVD player, thereby generating the reflected beam.

[0059] The reflected beam has various intensities in accordance with the surface shape of the medium 320. After the beam irradiated on the medium 320 is reflected, the reflected beam subsequently passes through the lens 318 and the micro-mirror array 314. Then the polarization direction of the reflected beam is changed by as much as λ/2, and the beam with its polarization direction altered is reflected by the polarization splitter 306 and finally displayed by the CCD 308.

[0060] The CD/DVD player can be implemented in the holographic digital data storage system by configuring the structure as cited above and by following the above-described operation.

[0061]FIG. 7 depicts a block diagram of a holographic digital data storage system compatible with a CD/DVD player in accordance with another embodiment of the present invention, wherein the holographic digital data storage system comprises a mirror 313 and a penetrative SLM 700 such as a penetrative LCD instead of the micro-mirror array 314 of FIG. 3.

[0062] The beam width can be adjusted by an on-off operation of the penetrative SLM 700, and the structure and operational method of the hologram digital data storage system in FIG. 7 is the same as that illustrated in FIG. 3 except the penetrative SLM 700 and the mirror 313.

[0063]FIG. 8A describes an optical path of the holographic digital data storage system with the penetrative SLM 700 such as the penetrative LCD. An active region of the penetrative SLM 700 is adjusted so that the CD/DVD player may be compatible with the holographic digital data storage system.

[0064]FIG. 8B explains an optical path when the position of the penetrative SLM 700 is changed. It is possible to enlarge input data capacity of the holographic digital data storage system by moving the penetrative SLM 700 into a region of the holographic digital data storage system.

[0065] When the active region of the penetrative SLM 700 is dislocated from the optical path, as shown in FIG. 8B, the intensity of the reconstructed beam is more reduced than that of the reconstructed beam in the embodiment shown in FIG. 8A. In such an event, a larger volume of data may be inputted by adjusting the beam size.

[0066]FIG. 9 is a block diagram of a holographic digital data storage system compatible with a CD/DVD player in accordance with another embodiment of the present invention. The holographic digital data storage system comprises a high frequency overlap module 10, four mirrors 11, 18, 905 and 907, a polarizing prism 12, a cylindrical lens 13, a charge coupled device (CCD) 14, a disc medium 16, an object lens 17, a collimating lens 19, three wave plates 900, 902 and 903 and a spatial light modulator (SLM) 904.

[0067] The first wave plate 900, the second wave plate 902 and the third wave plate 903 are disposed at an output terminal of the high frequency overlap module 10 and a forward reflection terminal and a transmission terminal of the polarizing prism 12, respectively. The SLM 904 is located between the object lens 17 and the mirror 18. The CCD 14 is located at the backward reflection terminal of the polarizing prism 12.

[0068] In the CD/DVD player mode, a numerical aperture may be adjusted by performing an on-off operation for all the pixels in the SLM 904 during the reproducing process. It is possible that the first wave plate 900 may not rotate the polarized beam while the third wave plate 903 functions as the λ/4 plate 15 shown in FIG. 3. Specifically, the third wave plate 903 is controlled in order that the beam reflected by the disk medium 16 should be reflected by the polarizing prism 12 to proceed to the CCD 14.

[0069] In the holographic digital data storage system, on the other hand, the polarized beam is rotated to a predetermined direction by the first plate 900, and then the beam is divided into two kinds of beam by the polarizing prism 12. The subsequent writing/reconstruction mechanism is identical with the conventional method. A reconstructed beam at the time of reproducing process proceeds to the opposite direction of the reference beam used for the reproducing. The second wave plate 902 for the reference beam may be adjusted in such a way that the reconstructed beam should transmit through the polarizing prism 12. The second wave plate 902 is preferably controlled to rotate the polarization of the reference beam by as much as 90 degree. The third wave plate 903 may not be rotated.

[0070] The disk medium 16 used in the holographic digital data storage system shown in FIG. 9 should have no reflection coating. Therefore, the holographic digital data storage system of the present invention may be compatible with the CD/DVD player, thereby reducing the cost.

[0071] While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims. 

What is claimed is:
 1. A holographic digital data storage system compatible with a CD/DVD player, comprising: a light source for generating a beam; a first wave plate for altering a polarization direction of the beam to a predetermined direction to generate a polarized beam; a beam expander, disposed to confront with the first wave plate, for expanding the size of the polarized beam; a polarization splitter for splitting the expanded beam into a vertical polarized beam, vertical to the polarization direction, and a horizontal polarized beam, horizontal to the polarization direction; a second wave plate for altering a polarization direction of the horizontal polarized beam to another predetermined direction to generate a third directional polarized beam; a first mirror for adjusting an optical path of the vertical polarized beam to generate a first path-controlled beam; a second mirror for controlling an optical path of the third directional polarized beam to generate a second path-controlled beam; a first and a second lenses for concentrating the first and the second path-controlled beams, respectively, to generate a first and a second concentrated beams; and a medium for recording the interference of the first and the second concentrated beams.
 2. The holographic digital data storage system of claim 1, wherein the beam is a laser beam.
 3. The holographic digital data storage system of claim 1, wherein the medium includes a substrate, a holographic digital data storage material and a mirror coating material sandwiched between the substrate and the holographic digital data storage material.
 4. The holographic digital data storage system of claim 1, wherein the second mirror is a micro-mirror array with a plurality of mirror pixels for forming a plurality of optical channels.
 5. A holographic digital data storage system compatible with a CD/DVD player comprising: a light source for generating a beam; a first wave plate for altering a polarization direction of the beam to a predetermined direction to generate a polarized beam; a polarization splitter for splitting the polarized beam into a reference beam and a signal beam; a second wave plate, located at a forward reflection terminal of the polarization splitter, for adjusting the reference beam so that a reconstructed signal beam of the signal beam in the holographic digital data storage system mode is transmitted through the polarization splitter; a third wave plate, located at a transmission terminal of the polarization splitter, for controlling the signal beam so that a reconstructed signal beam of the signal beam is reflected by the polarization splitter in the CD/DVD player mode; a spatial light modulator for adjusting numerical aperture by an on-off operation on a pixel basis in the CD/DVD player mode; a medium for recording the interference of the reference and the signal beams; and means for, located at the backward reflection terminal of the polarization splitter, displaying the reconstructed signal beam. 